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Case file 001 · An interactive investigation

The Re-identification Test

"We anonymize data" is the most common unverifiable claim in the industry. This page puts it on trial: a short explainer, then a working demo where four anonymous cookie jars assemble a full profile of a fictional person. Then, if you want, run it on your own cookies.

Cookie jars
4
Names in the data
0
Profiles assembled
1

100% synthetic demo data. Nothing on this page identifies a real person.

The claim nobody can check

"We anonymize data." Every ad-tech vendor says it. Almost none of them publish anything you could use to verify it: no list of quasi-identifiers, no k threshold, no linkage test results. Names and emails get stripped, the dataset gets called anonymous, and everyone moves on.

The problem is that names were never the hard part. The hard part is the boring fields: postal district, device fingerprint, browsing hours. Combine enough of them and rows become unique to one person, no name required.

A claim you cannot test is not a safeguard. It is marketing.

The test

The test is simple to state: take a dataset someone calls anonymous and try to re-identify it. The classic move is a linkage attack. Combine quasi-identifiers with outside data and count how many rows become unique to one person.

Two yardsticks make it reproducible. k-anonymity asks how many people share each combination of quasi-identifiers. If the answer is one, the data is not anonymous. l-diversity asks whether sensitive values stay mixed inside each group, so that even linked rows do not expose one person's secret. Same input, same parameters, same answer. Anyone can replicate it, which is exactly what makes it a test instead of a slogan.

What Law 25 changes

Under Quebec's Law 25, consent for non-essential cookies has to be real opt-in: manifest, free, enlightened, and given for specific purposes. Pre-ticked boxes and "continued browsing means consent" do not meet that bar.

Toggle the banner in the demo below and watch what changes. With the banner up and marketing cookies rejected, the jars hold only pseudonymous IDs with nothing to join on, and the profile collapses. Consent is not a popup. It is the difference between a profile and a pile of IDs.

What you are worth

Every jar in the demo carries a fictional bid price: what advertisers would pay per thousand impressions for that slice of a profile. The numbers are invented. The mechanism is not. Real-time bidding broadcasts quasi-identifiers to hundreds of vendors on nearly every page load.

Watch the total move as you toggle the banner. That gap, between the profiled price and the consented price, is the market value of your consent.

Try it yourself

Start with the fictional demo: zero risk, none of your cookies involved. The self-serve options below it ask you to agree to short terms first, because a cookie export can contain login sessions.

Start here

Walk the fictional demo

Four anonymous jars, one assembled profile. No terms to accept, nothing leaves your browser.

With your own cookies

  • Use my own cookies

    Paste an export, or collect jars site by site with the bookmarklet.

  • Collect the whole browser

    The Chrome extension grabs every site's cookies in one click.

    Get the extension

One-click collection

Collect the whole browser at once

The bookmarklet collects one site at a time. The companion Chrome extension analyzes every site's cookies in one click, inside the extension itself, and hands this page only per-site summaries: cookie counts, known tracker names, an exposure score, and a fictional bid value. Raw authentication tokens and cookie values never reach the page.

About the permission it asks for.Chrome will warn that the extension can read data on all websites. That broad wording is Chrome's, not a choice: reading every site's cookies is the entire job. The extension makes no network requests, runs no analytics, and sends your cookies nowhere. The only destination is this page, on your device.

Download the extension (.zip)
  1. Unzip the download.
  2. Open chrome://extensions and turn on Developer mode.
  3. Click "Load unpacked" and choose the unzipped folder.
  4. Click the new toolbar icon, then "Collect all cookies". This page opens with your jars queued behind the usual terms.

Manual install for now; a Chrome Web Store listing is planned. The zip includes the full source and a privacy note.

Exhibit A

The lab

Meet Jordan. Fictional person, fictional cookies, four fictional sites. Each jar below is "anonymized": hashed IDs, no names, no emails. The profile on the right assembles itself from what the jars share.

Simulates what a site where the visitor has an account already holds, and what happens when it joins the other jars.

Demo: toggling the Law 25 banner on collapses the assembled profile; toggling it off rebuilds it
Watch the profile collapse when the visitor rejects non-essential cookies, then rebuild when the banner is off. Fictional data.

Before you continue

  • This tool runs entirely in your browser. Nothing you paste is uploaded or transmitted anywhere.
  • The bookmarklet hands the current site's cookies to this page through the URL fragment, which browsers never send to servers. Only a per-site summary is kept in your browser; raw cookie values are discarded.
  • The optional browser extension analyzes your cookies inside Chrome and hands this page only per-site summaries: cookie counts, known tracker names, an exposure score, and a fictional bid value. Raw cookie values never reach this page, and nothing is uploaded anywhere.
  • A cookie export can contain login session tokens. Never share the file with anyone, and delete it when you are done.
  • Only analyze cookies from your own browsing, on your own device.
  • The profile it builds is a set of illustrative guesses, not verified facts about you.
  • This is an educational demo, not legal advice and not a security audit.

A note on what this is

An educational demonstration of re-identification concepts. It is not legal advice, not a compliance assessment, and not a finding about any real company or dataset. The demo figures are fictional. For decisions with legal consequences, talk to a qualified privacy lawyer.